In the simplest terms, the spine is a column made of vertebrae and discs. The vertebrae provide the support and structure of the spine while the spinal discs, located between the vertebrae, act as cushions or “shock absorbers.” These discs also contribute to the flexibility and motion of the spinal column. Over time, the discs may become diseased or infected, may develop deformities such as tears or cracks, or may simply lose structural integrity (e.g., the discs may bulge or flatten). Impaired discs can affect the anatomical functions of the vertebrae, due to the resultant lack of proper biomechanical support, and are often associated with chronic back pain.
Several surgical techniques have been developed to address spinal defects, such as disc degeneration and deformity. Spinal fusion has become a recognized surgical procedure for mitigating back pain by restoring biomechanical and anatomical integrity to the spine. Spinal fusion techniques involve the removal, or partial removal, of at least one intervertebral disc and preparation of the disc space for receiving an implant by shaping the exposed vertebral endplates. An implant is then inserted between the opposing endplates.
Spinal fusion procedures can be achieved using a posterior or an anterior approach. Anterior interbody fusion procedures generally have the advantages of reduced operative times and reduced blood loss. Further, anterior procedures do not interfere with the posterior anatomic structure of the lumbar spine. Anterior procedures also minimize scarring within the spinal canal while still achieving improved fusion rates, which is advantageous from a structural and biomechanical perspective. These generally preferred anterior procedures are particularly advantageous in providing improved access to the disc space, and thus correspondingly better endplate preparation.
Several interbody implant systems have been introduced to facilitate interbody fusion. Traditional threaded implants involve at least two cylindrical bodies, each typically packed with bone graft material, surgically placed on opposite sides of the mid-sagittal plane through pre-tapped holes within the intervertebral disc space. This location is not the preferable seating position for an implant system, however, because only a relatively small portion of the vertebral endplate is contacted by these cylindrical implants. Accordingly, these implant bodies will likely contact the softer cancellous bone rather than the stronger cortical bone, or apophyseal rim, of the vertebral endplate. The seating of these threaded cylindrical implants may also compromise biomechanical integrity by reducing the area in which to distribute mechanical forces, thus increasing the apparent stress experienced by both the implant and vertebrae. Still further, a substantial risk of implant subsidence (defined as sinking or settling) into the softer cancellous bone of the vertebral body may arise from such improper seating.
In contrast, open ring-shaped cage implant systems are generally shaped to mimic the anatomical contour of the vertebral body. Traditional ring-shaped cages are generally comprised of allograft bone material, however, harvested from the human femur. Such allograft bone material restricts restrict the usable size and shape of the resultant implant. For example, many of these femoral ring-shaped cages generally have a medial-lateral width of less than 25 mm. Therefore, these cages may not be of a sufficient size to contact the strong cortical bone, or apophyseal rim, of the vertebral endplate. These size-limited implant systems may also poorly accommodate related instrumentation such as drivers, reamers, distractors, and the like. For example, these implant systems may lack sufficient structural integrity to withstand repeated impact and may facture during implantation. Still further, other traditional non-allograft ring-shaped cage systems may be size-limited due to varied and complex supplemental implant instrumentation which may obstruct the disc space while requiring greater exposure of the operating space. These supplemental implant instrumentation systems also generally increase the instrument load upon the surgeon.
The surgical procedure corresponding to an implant system should preserve as much vertebral endplate bone surface as possible by minimizing the amount of bone removed. This vertebral endplate bone surface, or subchondral bone, is generally much stronger than the underlying cancellous bone. Preservation of the endplate bone stock ensures biomechanical integrity of the endplates and minimizes the risk of implant subsidence. Thus, proper interbody implant design should provide for optimal seating of the implant while utilizing the maximum amount of available supporting vertebral bone stock.
Traditional interbody spinal implants generally do not seat properly on the preferred structural bone located near the apophyseal rim of the vertebral body, which is primarily composed of preferred dense subchondral bone. Accordingly, there is a need in the art for interbody spinal implants which better utilize the structurally supportive bone of the apophyseal rim.
In summary, at least ten, separate challenges can be identified as inherent in traditional anterior spinal fusion devices. Such challenges include: (1) end-plate preparation; (2) implant difficulty; (3) materials of construction; (4) implant expulsion; (5) implant subsidence; (6) insufficient room for bone graft; (7) stress shielding; (8) lack of implant incorporation with vertebral bone; (9) limitations on radiographic visualization; and (10) cost of manufacture and inventory. Each of these challenges is addressed in turn.
End-Plate Preparation
There are three traditional end-plate preparation methods. The first is aggressive end-plate removal with box chisel types of tools to create a nice match of end-plate geometry with implant geometry. In the process of aggressive end-plate removal, however, the end-plates are typically destroyed. Such destruction means that the load-bearing implant is pressed against soft cancellous bone and the implant tends to subside.
The second traditional end-plate preparation method preserves the end-plates by just removing cartilage with curettes. The end-plates are concave; hence, if a flat implant is used, the implant is not very stable. Even if a convex implant is used, it is very difficult to match the implant geometry with the end-plate geometry, as the end-plate geometry varies from patient-to-patient and on the extent of disease.
The third traditional end-plate preparation method uses threaded fusion cages. The cages are implanted by reaming out corresponding threads in the end-plates. This method also violates the structure.
2. Implant Difficulty
Traditional anterior spinal fusion devices can also be difficult to implant. Some traditional implants with teeth have sharp edges. These edges can bind to the surrounding soft tissue during implantation, creating surgical challenges.
Typically, secondary instrumentation is used to keep the disc space distracted during implantation. The use of such instrumentation means that the exposure needs to be large enough to accommodate the instrumentation. If there is a restriction on the exposure size, then the maximum size of the implant available for use is correspondingly limited. The need for secondary instrumentation for distraction during implantation also adds an additional step or two in surgery. Still further, secondary instrumentation may sometimes over-distract the annulus, reducing the ability of the annulus to compress a relatively undersized implant. The compression provided by the annulus on the implant is important to maintain the initial stability of the implant.
For anterior spinal surgery, there are traditionally three trajectories of implants: anterior, antero-lateral, and lateral. Each approach has its advantages and drawbacks. Sometimes the choice of the approach is dictated by surgeon preference, and sometimes it is dictated by patient anatomy and biomechanics. A typical traditional implant has design features to accommodate only one or two of these approaches in a single implant, restricting intra-operative flexibility.
3. Materials of Construction
Other challenges raised by traditional devices find their source in the conventional materials of construction. Typical devices are made of PEEK or cadaver bone. Materials such as PEEK or cadaver bone do not have the structural strength to withstand impact loads required during implantation and may fracture during implantation.
PEEK is an abbreviation for polyetherether-ketone, a high-performance engineering thermoplastic with excellent chemical and fatigue resistance plus thermal stability. With a maximum continuous working temperature of 480° F., PEEK offers superior mechanical properties. Superior chemical resistance has allowed PEEK to work effectively as a metal replacement in harsh environments. PEEK grades offer chemical and water resistance similar to PPS (polyphenylene sulfide), but can operate at higher temperatures. PEEK materials are inert to all common solvents and resist a wide range of organic and inorganic liquids. Thus, for hostile environments, PEEK is a high-strength alternative to fluoropolymers.
The use of cadaver bone has several drawbacks. The shapes and sizes of the implants are restricted by the bone from which the implant is machined. Cadaver bone carries with it the risk of disease transmission and raises shelf-life and storage issues. In addition, there is a limited supply of donor bone and, even when available cadaver bone inherently offers inconsistent properties due to its variability. Finally, as mentioned above, cadaver bone has insufficient mechanical strength for clinical application.
4. Implant Expulsion
Traditional implants can migrate and expel out of the disc space, following the path through which the implant was inserted. Typical implants are either “threaded” into place, or have “teeth” which are designed to prevent expulsion. Both options can create localized stress risers in the end-plates, increasing the chances of subsidence. The challenge of preventing implant expulsion is especially acute for PEEK implants, because the material texture of PEEK is very smooth and “slippery.”
5. Implant Subsidence
Subsidence of the implant is a complex issue and has been attributed to many factors. Some of these factors include aggressive removal of the end-plate; an implant stiffness significantly greater than the vertebral bone; smaller sized implants which tend to seat in the center of the disc space, against the weakest region of the end-plates; and implants with sharp edges which can cause localized stress fractures in the end-plates at the point of contact. The most common solution to the problem of subsidence is to choose a less stiff implant material. This is why PEEK and cadaver bone have become the most common materials for spinal fusion implants. PEEK is softer than cortical bone, but harder than cancellous bone.
6. Insufficient Room for Bone Graft
Cadaver bone implants are restricted in their size by the bone from which they are machined. Their wall thickness also has to be great to create sufficient structural integrity for their desired clinical application. These design restrictions do not leave much room for filling the bone graft material into cortical bone implants. The exposure-driven limitations on implant size narrow the room left inside the implant geometry for bone grafting even for metal implants. Such room is further reduced in the case of PEEK implants because their wall thickness needs to be greater as compared to metal implants due to structural strength needs.
7. Stress Shielding
For fusion to occur, the bone graft packed inside the implant needs to be loaded mechanically. Typically, however, the stiffness of the implant material is much greater than the adjacent vertebral bone and takes up a majority of the mechanical loads, “shielding” the bone graft material from becoming mechanically loaded. The most common solution is to choose a less stiff implant material. Again, this is why PEEK and cadaver bone have become the most common materials for spinal fusion implants. As noted above, although harder than cancellous bone, PEEK is softer than cortical bone.
8. Lack of Implant Incorporation with Vertebral Bone
In most cases, the typical fusion implant is not able to incorporate with the vertebral bone, even years after implantation. Such inability persists despite the use of a variety of different materials used to construct the implants. There is a perception that cadaver bone is resorbable and will be replaced by new bone once it resorbs. Hedrocel is a composite material composed of carbon and tantalum, an inert metal that has been used as a material for spinal fusion implants. Hedrocel is designed to allow bone in-growth into the implant. In contrast, PEEK has been reported to become surrounded by fibrous tissue which precludes it from incorporating with surrounding bone. There have also been reports of the development of new bio-active materials which can incorporate into bone. The application of such bio-active materials has been limited, however, for several reasons, including biocompatibility, structural strength, and lack of regulatory approval.
9. Limitations on Radiographic Visualization
For implants made out of metal, the metal prevents adequate radiographic visualization of the bone graft. Hence it is difficult to assess fusion, if it is to take place. PEEK is radiolucent. Traditional implants made of PEEK need to have radiographic markers embedded into the implants so that implant position can be tracked on an X-ray. Cadaver bone has some radiopacity and does not interfere with radiographic assessment as much as metal implants.
10. Cost of Manufacture and Inventory
The requirements of spinal surgery dictate that manufacturers provide implants of various foot-prints, and several heights in each foot-print. This requirement means that the manufacturer needs to carry a significant amount of inventory of implants. Because there are so many different sizes of implants, there are setup costs involved in the manufacture of each different size. The result is increased implant costs, which the manufacturers pass along to the end users by charging high prices for spinal fusion implants.